Transmission control in two-way signaling system



Jan. 5, 1937. B. G. BJORNSON ET AL 2 066,325

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Oct-22, 1935 FIG. .4 6 5.4 a

WEST EAST 154- I; N, DELAY 5 2 DELAY W4 4 I 1:

F/GJ asir m4 UREA AMP . B. a. BJoR/vso/v INVENTgiSM W ERM /7.

AT TORNEV Patented Jan. 5, 1937 UNITED stares FATE.

. TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Bjorn G. Bjornson, New York, N. Y., and Newton W. Bryant, Lyndhurst, N. 5., assignors to Bell Telephone Laboratories,

Incorporated, New

6 Claims.

The invention relates to two-way signaling systems and particularly to the signal-controlled switching circuits employed for directionally controlling signal transmission in such systems.

An object of the invention is to improve the operation of such circuits.

To obtain proper operation of long two-way signaling systems, such as four-wire toll telephone systems, it has been found necessary in the past to make use of voice-operated switching apparatus, so-called echo suppressors or anti-singing devices, for inserting a suitable loss in the signal path for one direction when signal transmission is taking place in the opposite direction so as to prevent echoes or reflected current from being transmitted back to the transmitting end of the system in sufficient volume to cause a disturbance or singing. Such a suppressor usually comprises an amplifier-rectifier circuit connected to the two-wire path for each direction and responsive to signal transmission therein to control the operation of mechanical relays to shortcircuit or open-circuit the signal transmission path for the opposite direction, or to suitably l control the operation characteristics of an amplifier therein.

In one type of circuit, the suppressor units are located at or near the terminals of the fourwire circuit. When such a suppressor is controlled from the output of the talking path, it

is known as a receiver terminal echo suppressor; and when operated from the input of the talking path it is known as a sender terminal echo suppressor. In both cases, it is desirable a to provide auxiliary means, the so-called hang over circuit, for prolonging the operation of each suppressor unit the proper interval of time after the supply of controlling speech waves thereto ceases in order to completely suppress all echoes of these speech waves and to prevent false operation of the other suppressor unit. With a receiver terminal type suppressor it has been necessary in the past to use artificial delay networks in the transmission path to prevent the transmission of the initial part of the signal preceding the application of the suppression loss to the other transmission path, which would cause false operation of the suppressor associated therewith. Since this short impulse would apply full 50 hang-over serious mutilation of the speech signal would result. Such electrical delay networks are relatively costly and often necessitate additional amplification to compensate for the loss they insert in the transmission path.

55 Experiments on terminal echo suppressors have shown that, due to side tone and air transmission, the talkers ear is highly desensitized during the utterance of a syllable and recovers its normal sensitivity gradually thereafter. The echo suppressor circuits of the present invention take advantage of this desensitization of the ear in such a way as to improve the operation under noise conditions. The circuits operate to insert a low loss in the echo path during the passage of the echo due to the initial part of a speech syllable and a larger loss thereafter in order that the effective echo may be maintained low when the ear recovers its sensitivity. No hang-over is made effective until the high loss is inserted. In one embodiment, the large loss is inserted in the echo path and the hang-over is applied only when the amplitude level of the transmitted signals exceeds a predetermined value. In another embodiment, a small loss proportional in value to the amplitude level of the transmitted signal is initially inserted in the echo path, and, if the input signal persists for a given interval of time, a large disabling loss is then inserted and hangover applied. These circuits can be operated satisfactorily without electrical delay circuits in the signal transmission paths.

The objects and advantages of the invention will be better understood from the following detailed description thereof when read in connection with the accompanying drawing in which:

Fig. 1 shows diagrammatically a four-wire toll telephone circuit equipped with transmission control circuits in accordance with this invention; and

Fig. 2 and Fig. 3 show in greater detail difierout constructions of the transmission control circuits-utilized in the system of Fig. 1.

The diagram of Fig. 1 is not an actual circuit diagram but rather a single line layout, each line indicating a transmission path. A normal make ma path is indicated by contacting arrow heads. An arrow directed at a make point indicates that the path will be disabled at that point by an associated voice-operated device.

The four-wire telephone circuit of Fig. 1 comprises a one-way transmission path EA including the one-way amplifying devices AI and A2, for repeating telephone currents in the direction from west to east between a two-way circuit terminated by the west station SW and a two-way 5 circuit terminated by the east station SE, and the one-way transmission path WA including the one-way amplifying devices A3 and A4, for repeating telephonic currents in the direction from east to west b tween the same two-way circuits.

The oppositely directed one-way transmission paths EA and WA may be connected at their terminal in substantially conjugate relation with each other and in energy transmitting relation with the two-way circuits, respectively, leading to the west and east stations in any suitable manner, for example, as indicated by hybrid coil transformers H and H2 and associated balancing networks N1 and N2.

Connected to the path EA near its output is the input of a voice-operated switching device or receiver terminal echo suppressor unit ES1 which operates in response to signal waves transmitted over the path EA to first insert a small transmission loss in the input of path WA for example, as indicated, by removing the normal short circuit around the resistance and subsequently, as indicated by the delay means 2 in one output branch of the suppressor ES1, to disable the path WA or to insert a large loss therein at a point 3 near the east terminal of the four-wire circuit. Similarly, connected to the path WA near its output is the input of a voice-operated switching device ESz which ope-rates in response to signal waves transmitted over the path WA to first insert a small loss in the input of the path EA, as by removing the normal short circuit around the resistance 4, and subsequently, as indicated by the delay means 5 in one output branch of the suppressor ESz, to disable the path EA or to insert a large loss therein at a point 6 near the west terminal of the four-wire circuit.

A preferred construction of the echo suppressor units ES1 and ESz is shown in Fig. 2. As indicated, each echo suppressor unit ESi or ESz comprises an amplifier the input circuit of which in the system of Fig. 1 for the case. of ESi would be connected to the one-way path EA at somepoint 8 near its output and for the case of ES2 would be connected to the one-way path WA at some point 9 near its output. The output of the amplifier l is coupled to the input of a threeelectrode electric discharge detector tube ID, the plate circuit of which includes in series the winding of relay M, an operating winding 2 of relay l3 and plate battery l4. A condenser I5 is connected directly between the plate of the tube l8 and the positive terminal of plate battery M, for

the purpose of diverting the alternating current components of the detected signals from the winding of relay II and the winding l2 ofrelay l3.

The winding of a third relay I6 is connected in a closed circuit through ground in series with resistances I! and I8 and a battery IS. The battery I9 is normally short-circuited through the resistance l8, the normally closed back contact and armature of relay II and ground, so that relay I6 is normally deenergized. The normally open front contact of relay II is connected in series with the winding of a fourth relay 23, battery 24 and ground so that when relay operates and shifts its armature from its back toits front contact the winding of relay 23 is energized by battery 24 over a circuit extending from the positive terminal thereof through the relay winding, the resistance 25, front contact and armature of relay II and ground to the negative terminal of battery 24.

The normally open contact of relay 23 is connected through series resistances 28 and 29, winding 26 of relay l3 and battery 21 to ground, and the junction point of series resistances 28 and 29 is connected through the condenser 38 to ground. When relay 23 operates the winding 26 of relay 3 is energized by current from battery 21 over a circuit extending from its positive terminal through the relay winding, resistances 28 and 29, closed contact and armature of relay 23 and ground to the negative terminal of battery 21.

.A loss network comprising the two tandem transformers 20 and 2|, is connected in each one-way path WA or EA near its input, for example, at the point 3 in the former and at the point 6 in the latter path inv the system of Fig. 1. The transformerzp has two secondary windings 3| and 32, and the transformer 2| has two primary windings 33 and 34. The upper terminals of windings 3| and 33 are connected together and likewise the lower terminals of windings 32 and 34. The lower terminal of winding 33 is connected to the upper terminal of winding 32, and the upper terminal of winding 34 is connected to the lower terminal of winding 3|. These connections taken by themselves are such that the windings 3|, 32, 33, and 34 would effectively oppose each other so as to prevent transmission through the network over the transmission path in which it is connected. However, as indicated, a normal cross-connection made through the closed armature and contact 7 of relay I6 and the closed armature and contact of relay IS in series ties together the lower terminals of the windings 3|, 33 and the upper terminals of windings 32 and 34, so that, in effect, two series aiding circuits are established through transformers 20, 2| and transmission through the transformer network with little attenuation is normally permitted.

A resistance 22 of small value, which corresponds to the resistance or 4 in the system of Fig. 1, is connected between the lower terminals of windings 3| and 33 and between the upper terminals of windings 32 and 34, in series with the normally closed contact and armature of relay I 3, but this resistance is normally shortcircuited by the normally closed armature and contact of relay l6. Thus, when relay lfi operates to break. this short-circuiting connection and relay l3 remains unoperated, the resistance,

22 is in series with the path WA or EA between the transformers 20 and 2| and introduces a corresponding small loss in the path. The subsequent operation of relay l3 in response to the shown in Fig. 2 when used as the echo suppressor units E81 and E32 in the system of Fig. 1 follows:

It will be assumed that the speech waves of a west subscriber at station SW are received over the two-way circuit associated therewith and are impressed by the hybrid coil transformer H1 on the input of the path EA, and, at that time, no speech waves in the direction from east to west are being received by the four-wire circuit over the two-Way circuit associated with the east station SE. Both echo suppressor units ES1 and ES: are normally in the unoperated condition indicated in Fig. 2 so that the loss network comprising transformers 20 and 2| in the aoceg'sas input of each transmission path EA and WA ofiers little or no loss to the transmission of signals. Therefore, the speech waves in the path EA after amplification by the amplifier-AI are transmitted out over the path lilA towards the east end of the four-wire circuit.

At the east end of the four-wire circuit, the main portion of the incoming speech waves will be amplified by the amplifier A2 and the amplified waves impressed by hybrid 0011 H2 on the associated two-way circuit over which it will be transmitted to the listening subscriber at the east station SE.

A small portion of the incoming speech waves, however, will be diverted from thepath EA into the input of the suppressor 'uni t E81 :at the point 8, and (see Fig. 2) will be amplified by the amplifier l and detected by the detector tube H3 therein. The resultant flow of current in the plate circuit of the detector tube l0 will pass through the winding of the relay H and the winding [2 of relay 13 in series. Relay H is designed to be operated immediately by this initial flow of current, to shift its armature from the back to the front contact. As soon as the armature of relay H leaves the back contact the short-circuiting connection around battery 19 is removed and energizing current is supplied thereby to the winding of relay it which operates immediately to attract its armature, removing the short-circuiting connection around the resistance 22. Resistance 22 is then effective to insert a small amount of loss corresponding to its value in the input of the path WA due to its connection to the windings of transformers 29 and 2!, in the manner which has been 'described previously. No hang-over is appiied to the relay it so that it releases immediately :at the termination of each signal impulse, reapplying the short circuit around resistance 22.

The small loss momentarily inserted in the input of the path WA by resistance $22 by each speech impulse will attenuate slightly the initial echoes thereof impressed by hybrid coil H2 on the path WA and passing through the transformer network 29, 2! during this interval. The attenuated echo currents will be transmitted to the west end of the path WA and after amplification by amplifier A i will be impressed on hybrid coil H2. The portion of the echo returned to the talker at station SW over the associated two-way circuit although of fairly high amplitude will not be troublesome because it is returned during the initial time interval, to, when the talkers ear is highly desensitized by speech.

Should the portion of the echo currents over the path WA be of suflicient magnitude to cause false operation of the echo suppressor unit E'Se connected to path WA, in the manner which has just been described for the corresponding echo suppressor unit ESi at the east end of the circuit, so that a small loss is inserted in the input of the path EA, this will cause only an inappreoiable degradation in the quality of speech transmission over the line EA. This is because the condition can exist only momentarily due to the short time that the relay of the suppressor ESz, corresponding to the relay 1%: of the suppressor ESi at the east end of the circuit, which inserts the loss remains operated before -operation of the relay corresponding to relay l3 oi suppressor ESi to remove the loss, and the echo currents thereafter received by ESz will be of too low amplitude to cause false operation because of the large loss inserted in the echo path at the east end of the circuit by the suppressor ESi after the initial time interval, as 'will be described.

Returning to the echo suppressor E31 :at the east 'en'd'of the four-wiresy'stem, the same speech impulse which causes the energization of the winding of relay H also causes the energization of the winding l2 of relay [3 in series therewith. The latter relay is a marginal relay designed to operate immediately whenever the level of the detected signal builds up, say L2 decibels, above the marginal level required to operate relay H, so that the relay l3 will operate after relay H, but within the initial time interval, and only if the applied signal reaches the required .level. The operation of relay l3 opens its armature and contact breaking the cross connection through the resistance 22 between the windings 3-1, 32, '33, and 34 of the transformers 20 and 21., producing in the manner which has been described a differential connection of these windings and therefore the insertion 'of a large :loss in the input of the path WA at the point 3.

If the relays in echo suppressor ESz at the west end of the four-wire circuit, corresponding to the relays H and it of the echo suppressor ES1 at the east end, had previously been falsely operated in response to the initially transmitted slightly attenuated echo currents, because of the insertion of the large loss in the path WA at the east end of the circuit cutting off subsequently the transmission of echo currents of appreciable amplitude, they will quickly release.

When the relay H in the echo suppressor ES1 at the east end of the four-wire circuit is 'completely operated in response to the initial speech impulse, that is, when its armature closes through the front contact of the relay, the winding of relay 23 is energized by battery 25 over a circuit extending from the positive terminal of the battery, through the winding of relay 23, resistance 25, front contact and armature of relay H and ground to the negative terminal of the battery. The winding of relay 23 is shunted by a condenser 51, the charging of which by current from the battery -24 slows up the operation of relay 23. When relay 23 operates, it closes through its armature and contact an energizing circuit for the hang-over winding 26 of relay [3 from battery 2! which may be traced from the positive terminal of the battery through winding 26, resistances 28 and 29 in series, closed armature and contact of relay 23 and ground to the negative terminal of battery N. If relay is has operated in response to the initial impulse through its winding [2, the energiz-ation of winding '26 maintains the relay it operated, for winding 26 is wound so as toassist the action 'of winding l2. However, if the amplitude level of the detected speech pulse applied to winding 12 is not sufiiciently high to cause its complete energization, the energization of winding 25 will effect the operation of relay l3. Hang-over is applied to relay I3 When the winding 26 is energized but does not become eiiective unless and until relay is is operated. Relay I3 when operated, as explained above, breaks the connection of resistance 22 to the windings of transformers 2i! and 2! located at the point 3 in the path WA thereby.

inserting a large loss in that path.

When the winding i2 of relay I3 is deenergized at the end of the speech impulse causing its operation, relay l3 will not immediately release but will be maintained operated for an additional hang-over time interval while condenser 30, which was discharged to ground when relay 23 operated, is being charged by current from battery 21, through the winding 26 and resistance 28. When condenser 30 is completely charged by battery 2'1, the flow of current through winding 26 will cease due to the previous release of relay 23 and the armature and contact of relay 13 will return to their normally closed condition. At this time relay l6 has also released and resistance 22 is short-circuited through its armature and contact so that the windings of transformers 20 and 2| in the path WA are connected as described above in such manner that the loss inserted in the path WA is very low. The next received speech syllable repeats the operation which has just been described.

The values of the elements in the hang-over circuit for relay [,3 above described are made such as to insure that relay l3 remains operated for a sufficient time after the end of the controlling speech syllable to maintain the high loss in the input of the path WA until all echoes thereof have been substantially dissipated by that loss. The high loss inserted in the path WA also serves to block the transmission of subsequently received east subscribers speech currents over that path while the west subscriber continues to talk and thus prevents false operation of the echo suppressor unit ES2 at the west end of the circuit to reverse control of the circuit during this time interval.

The operation of the system for the case where the east subscriber is the first to start talking is similar to that described above for the case where the west subscriber starts talking first. The east subscribers voice currents are impressed by hybrid coil H2 upon the path WA and transmitted thereover with a certain amount of amplification produced by amplifiers A3 and A4 to station SW. A portion of each syllable of the east subscribers speech current is diverted from the path WA at the point 9 near the west end of the circuit and causes operation of the echo suppressor ES2, connected thereat to insert a small loss in the inputs of the path EA during the initial interval while the speakers ear is greatly desensitized and a larger loss in that path thereafter, the latter loss being maintained in the path EA for a sufiicient time to completely suppress the echo currents due to each syllable of the east subscribers speech currents. As in the case of the echo suppressor ES1 at the east terminal just described, the relay circuits of the suppressor ESz act to prevent the larger loss being inserted in the path'EA and the hang-over from becoming efiective for a definite period unless the input signal reaches the required level.

Somewhat difierent operation may be attained with the circuit of Fig. 2, if the windings l2 of relay 13 are omitted, the windings of relay H and condenser l alone being connected in the plate circuit of tube ID, as indicated by the dotted line X in the figure, and relay I 3 is designed to be operated to insert the large loss in the echo path when the winding 26 of the'relay is energized by currentfrom battery 21 in response to the operation of relay H and subsequent operation of relay 23. The operation of the modified circuit differs from that of the circuit of Fig. 2 in that, if the detected impulse in the output of detector It does not persist above the marginal or just operate level for at least a time h, the time required to complete the energization of the winding 26 of relay l3, the relay 13 does not operate and no hang-over is applied. If the input signal persists for the required time, the relay l3 operates, inserting the large loss and the associated hang-over. This time interval may be made of any desired length by proper design of the relays ll, 23, and I3 and the associated resistances and condensers. The time is made such that the relay 13 will be operated by a speech syllable but will be unoperated by the usual noise impulses.

Fig. 3 shows a modified construction in accordance with the invention which may be used for the echo suppressor units ES1 and E82 in the system of Fig. 1. This modification makes use of the so-called proportional losser suppressor, illustrated conventionally as the variable gain amplifier 35 and a circuit 36 for reducing its gain in proportion to the increase in amplitude level of the Waves in the output of the amplifier 38.

One example of such a proportional losser circuit which might be used is illustrated in detail in Fig. 2 of the Patent 1,840,015 issued to B. G. Bjornson on January 5, 1932, wherein the gain of a vacuum tube amplifier in one amplifying path of a four-wire circuit is continuously reduced in proportion to the rise in amplitude level of the waves in the other amplifying path after a certain threshold value is reached, by a control circuit employing no movable elements. Other types of proportional lossers which may be used are disclosed in A. B. Clark Patent 1,829,928 is.- sued December 8, 1931, and 1,745,457 issued February 4, 1930.

As indicated, the variable gain amplifier 35 would be connected in the input of the path WA or EA in front of the transformer network 31 employed for producing the large disabling loss in the path and which is similar to the trans- Silent Patent former network 20, 2| described in connection the case of the suppressor ES1, or across the path WA at the point 9 in the case of suppressor ESz.

A transformer 40 connects the circuit coupling the output of the amplifier 38 to the input of the control circuit 36 to the input of a threeelectrode vacuum tube detector tube 4|, the plate circuit of which includes the winding of a relay 42 in series with the plate battery 43. Condenser 44 is connected across the plate circuit of tube 4! in shunt with the winding of relay 42, and serves to keep the alternating current components of the detected currents out of the relay winding. The winding of a second relay 58 is normally energized by current from the battery 45 over a circuit extending from the positive terminal of the battery through the winding of relay 58, series resistances 46 and 41, normally closed contact and armature of relay 42, and ground to the negative terminal of battery 45, so as to maintain its armature and contact separated. A condenser 48 is connected between the junction of resistances 46 and 41, and ground.

The winding of a third relay 49 is adapted to be energized by current from the battery 50 when the normally operated relay 58 releases to close its contact and armature, over a circuit extending from the positive terminal of battery 50, through winding of relay 49, series resistances 5| and 52, closed contact and armature of relay 58 and ground to the negative terminal of battery 58. A condenser 59 is connected between the junction of the series resistances 5| and 52 and 10.: armature, the. windings'l, 52,53, and 54 of the 203pressor- E81, or'at the point accesses- 5 in connection with the similar network 20, 2] in the system of Fig. 2, so that. normally the: network 31; offers little. or notransmission loss in thepath WA or EA. in- Whlchritz is connected; and when relay. 49 operates to" open its. contact and transformer network are connected in opposing relation so that a:larg,e:loss-:suflicient toprevent appreciable transmission through the network: is inserted in the path. WA or EA.

inthesystem of Fig. 1,.the circuitarrangement of I-Fig. 3. operates as follows: Thewave impulses impressed. on the input. of: the suppressorat the point8 in the. path. EAzin the case" of the sup- 9 in the'path WA inthe case'ofi thesuppressor. ESz', are amplified by the amplifierr 38 and'the amplified-.wavesin the output-of. amplifier 38'are divided between the input of the control circuit 36 and theinput 28;- of; the detector 4|. The: portion impressed on the input of the control circuit'36' afteracertain threshold .value of level is reached will cause thecontrol circuit 36 tooperate to reducethe amplification of. the variable'gainamplifier 35 in the path WA .or- EA .inproportion to the: amplitude of thewaves' impressed: on the input of: the sup.- pressor,v thus. eliectivelyinserting, in the. transmission: path WA: or EA: a proportional loss' in series'with the network 31.

By suitable design: of i the: proportional losser circuit, thevalue of this initial loss is made small but. such that the echocwavedue to the initial portion-of each speech syllable inserting the loss transmitted to the talkinggsubscriber-at the other 4Q; terminal .of. the system during the initial time interval while his-ear:ishighlyjdesensitized by speech; is of sufiicientlylowamplitude as not-to be. troublesome; and such.- as to prevent false operation of;the.=echo*suppressorat the trans-1 mitting terminal by; the received? echo wave.

of a speech syllable.

chain insert The. noise. waves in -tl1e-.path :EAwr: WA during the time in which-speech is not beingatransmittedwill also insert "a loss proportional: to'the noise levelin theother path. By suitable design of the proportional losser circuit,- this'loss maybe made-at any instant just sufficient toprevent falsev operation of the associated suppressors'by.- noise, thereby allowing the suppressors to be maintained 1 at maximum sensitivity as- :regards operation by speech.v

The portion of the outputcurrent ofthe amplifier 38 in the system of Fig. 3; impressed-on the. input circuittof detector tube 4| -by transformer 40 will be detected thereby and-supplied as energizing current to the windingof -relay.42

which is thefirst relay. in the chain which, when completely operated, causes a large disabling loss to be inserted inthe echo path through'the-transformer network 31. It is desired that this relay the large disabling loss in the'echo path and apply hang-over only if "theinput signal applied 'to the detector 'circuitpersists'above 'a given level for "a predeterminedtime t2, corresponding' to that which would obtain inthe case Preferably, the large loss should be inserted in the echo'path and the hang overapplied before-the 'talkers ear has regained its -normalsensitivity.- This'type of operation is' obtained by suitable design of the resistancecondenser combinations 46; 41, 48 and-5 I 52, l 59 When used as an echo suppressor ES1 orESz associated'with therelays 58' and 49, respectively.

Relay 42. is operated immedately by the detected currents, whether due to speech or noise, t'o'break-its armature and contact and thus break the energizing circuit. for the winding of relay 58* for battery 45 through theresistances 46, 41 and ground. However, relay 58 will not release immediately but will be retained operated for a definite'time' thereafter while the condenser 48 is: beingicharged upby current from battery 45 through the winding of relay 58 and resistance 46;. When condenser '4'8 is charged up, the relay 58:wilb release closing. its armature and contact thereby closing an energizingcircuit for the winding of. relay larfromibattery 50 through resistances. 5i and- 523. closed armature and contact. of: relay." 58* and ground, and causing condenser: 59*"normallycharged by current from battery 5ll tobeidischargeda If the applied signal does not persistz for the required interval of time, t5, the chaimof operation will be broken at some poin't',.the. windingot": relay 49"will not be completely energized; and-hang-over will not be applied tol'that" relay.

Ifathelevel of. the appliedsignal persists for the required time interval, .relay. 49 is' energized and? operatesT-tobreak. its contact thereby conneotin'g. thewindings 51, 52, 53 and. 540i the transformer network 31 in opposing relation so that the network inserts effectively in the echo path av largedossito supplement the small loss previouslyv inserted'in' the echo path by the proportional lossercircuit36'=,35. Any echoes transmittedto the talker at: station SW after his ear recovers its normal-sensitivity, due to this large loss,.are"of'such amplitude as not to be troublesome;

At the end of eachitransmitted speech syllable, or when the talking subscriber ceases talking; the" relay 42. will release'but the last relay" in the chain, relay 49;.will bez maintained operated for an additional hang-over interval while first condenser 4a-1s= being discharged to ground throughathe resistance 41 and then while condenser 59 is being charged by current from bat tery 50 through the winding'of relay 49 and resistance'51; By suitable-Idesignof the circuit elements, thishan'g-over' interval ism'ade suffioiently. long to maintain relay 48- operated to maintain the largeloss in the echo path through network 3lsuntil all echoes of the talkers controlling speech currents are dissipated thereby. At the end of thisinterval, relay- 49 will release and the networkr3'l will-be returned to its nor-1' mallow loss condition.-

Tests of 'each of the abovedescribed echo suppressor circuits in connection with four-wire telephone circuits indicate that the optimum values'of'the small loss Li, inserted in the echo path duringflthe" initial" period, to, for proper suppression ofeohoes depends on the transmission time of the-circuit and the suppressor sensitivity. For short transmission times, -L1= may be made small and to quite large. It wasiound that when the round trip-transmission time is quite small, itis possible' to obtain' proper suppression of "echoes "for L'1 'zero. In generaL it' may be said that when- LLiS made small, to must be short, but if L1--i$ made-large,to maybe made-"- long.

Each of the circuits of' the invention which have beendescribe'd' above will perrilit noise 'hav-' ing peaks above the marginal" sensitivity of 'thesuppressorof 'duration'less than to' without se' rious degradationin-the quality of transmission.

In the circuit of Fig. 2, noise spurts above the operate point, lasting less than time to and never reaching an amplitude of L2 decibels above the operate amplitude will put only the small loss L1 in the transmission path. However, if the noise amplitude becomes greater than L2 decibels above the marginal or operate amplitude, maximum loss is immediately inserted in the echo path. This function reduces the false operation of the other suppressor by echoes of speech. V

In the modification of the circuit of Fig. 2 described above in which the Winding l2 of the relay I3 is omitted and the winding 26 of the relay controls the insertion of the large .loss in the transmission path, noise spurts above the operate point of the suppressor circuit and of duration less than to will. never insert a loss more than L1 decibels in the transmission path regardless of the amplitude of the spurts. The number of times that the loss L1 is inserted and the intervals of time it is inserted are equal to the number of times the noise spurts rise above the operate point and the lengths of time they remain above, respectively. These insertions of loss in the transmission path will degrade the transmitted speech somewhat, but tests indicate that the degradation will be practically negligible on some types of noise.

The operation of the circuit of Fig. 3 on noise is somewhat similar to that of the circuit of Fig. 2, but the small loss L1 inserted in the transmission path is variable, its instantaneous magnitude depending on the instantaneous amplitude of the noise spurts. Any noise spurt lasting less than to second will insert loss proportional to its amplitude. If it lasts longer than to second, the large loss wil be inserted and proper hang-over applied.

Various modifications of the circuits of the invention which have been described and illustrated. withinthe scope of the invention will occur to persons skilled in the art. The invention is only to be limited by the scope of; the appended claims.

What is claimed is:

1. In a two-way telephone transmission system comprising at least near the terminals thereof two associated one-way transmission paths for transmitting the telephonic signals in opposite direction, switching means connected to one one-way path near one terminal and responsive to speech signal transmission in said one path to insert a loss of small value in the other one-way path suflicient to attenuate the echoes of the speech signals a relatively small amount during an initial time interval of a length corresponding to the time during which a talkers ear is relatively desensitized by the speech signals and to insert a loss of higher value sufficient to substantially suppress echoes of the speech signals in said other path at the end of such interval.

2. In a two-way telephone transmission system co-mprisng two associated one-way transmission paths for repeating signals in opposite directions, a switching device connected to one oneway path and responsive to speech wave transmission therein to insert a loss of small value at a point in the other one-Way path while an echo due to the initial part of a speech syllable is passing that point therein and to insert a larger loss in said other path thereafter, means for producing a hang-over in the operation of said switching device so that said larger loss will be maintained in said other path a desired interval of time after cessation-in the supply of controlling energy thereto, and means for preventingv opposite directions between two two-way lines,

a voice-operated switching device connected to each one-way circuit near its output and responsive to incoming speech signals therein tor insert a loss of small value at a point in the other one-way circuit while an echo due to the initial part of a syllable of said signal is passing that point in said other circuit, and to insert a disabling loss thereafter in said other circuit, means for producing hang-over in the operation of said switching device for a desired time interval after the supply of controlling energy thereto ceases, and means for preventing'said hang-over from becoming effective until said disabling loss is inserted in said other one-way circuit.

4. In a two-way telephone system comprising two associated one-way paths for repeating telephone signals in opposite directions, a voiceoperated switching device connected to one oneway path and immediately responsive to the initial low amplitude part of each syllable of the speech signal waves receivedtherefrom to insert a loss ofsmall value at apoint in the other one-way path while an echo due to the initial part of said syllable is passing that point therein, said loss being suflicient to produce only a small amount of attenuation in said echo and'responsive to the higher amplitude parts of each received speech syllable to insert a large disabling loss in said other one-way path, and means for device connected to each one-way path near its output and operatively responsive to incoming speech signals therein to insert a loss of small value at a point in the other one-way path near its input while an echo due to the initial part of a syllable of said signals is passing that pointin said other path, and responsive to incoming i speech signals of a level slightly higher than the marginal operating level of said device to insert a large disabling loss in said other path near its input, means for producing-a. hang-over in the operation of said switching device for a desired time interval after the end of the controlling speech syllable, and means responsive to operation of said device to prevent said hang-over from becoming efiective unless and until said large disabling loss is inserted in said other path.

6. In a two-way telephone system, a four-wire circuit comprising twooppositely-directed oneway paths for repeating telephonic signals in opposite directions between two two-way transmission circuits, a wave-operated switching de-' to the increase in level of the controlling waves, and responsive to an impressed wave impulse of level above said threshold value only when it persists for a given length of time, to insert a large disabling loss in said other path near its output, means responsive to operation of said device for producing a hang-over in its operation a definite time after the end of the controlling impulse, and means for preventing said hangover in operation from becoming efiective unless and until said large loss is inserted in said other path.

BJORN G. BJORNSON. NEWTON W. BRYANT. 

